Avoid if statements for checking neighboring indexes in a 2D array

[Timon Gehr via Digitalmars-d-learn]

On 16.07.2017 12:37, kerdemdemir wrote:
> My goal is to find connected components in a 2D array for example 
> finding connected '*'
> chars below.
> 
>        x x x x x x
>        x x x x x x
>        x x * * x x
>        x x * * x x
>        x x x * * x
>        * x x x x x
> 
> 
> There are two connected '*' group in this example. First group is 
> composes of six '*' located closer to middle and the second group 
> composes only one '*' char located in the left bottom corner.
> 
> Do to this I generally implement a recursive algorithm which repeat 
> calling the same function by checking all neighbors around the current 
> index. I generally end up with something like :
> 
> void foo( int row, int col)
> {
>      //Do something here like caching the index
> 
>      if ( twoDimensionData[row - 1][col] == '*')
>         foo(row- 1, col);
>      else if ( twoDimensionData[row + 1][col] == '*')
>         foo(row+ 1, col);
>      else if ( twoDimensionData[row - 1 ][col - 1] == '*')
>         foo(row - 1, col - 1);
> 
> //..... I need 5 more of this bad boys I mean if checks
> }
> ...

It is wrong to explore in only one direction, so I assume you do not 
mean "else".

> Is there any better way to achieve this
foreach(i;row-1..row+2){
     foreach(j;col-1..col+2){
         if(i==row && j==col) continue;
         if(twoDimensionData[i][j] == '*')
             foo(row,col);
     }
}

> with cool std functions like 
> enumerate or iota without needing to write eight if checks?

cartesianProduct(iota(row-1,row+2),iota(col-1,col+2))
     .filter!(a=>(a[0]!=row||a[1]!=col))
     .filter!(a=>twoDimensionData[a[0]][a[1]]=='*')
     .each!(a=>foo(a.expand));

(You can usually drop the first filter because "doing something" will 
usually involve checking if the node has been visited and returning or 
else marking the node as visited.)